Bus structures are widely used for interconnecting printed circuit boards in digital systems such as computers and switching systems. In bus systems, signals are commonly connected to each circuit board in the system. The load and parasitic capacitances of each circuit board and interconnection add to the load on each driver circuit. As a result, for each additional circuit board, additional drive capability is required to prevent reduction of operating speed. In addition, the bus operating speed is limited by the time to transmit signals over the maximum length of the bus.
Bus interconnection systems are satisfactory for many applications. However, when it is necessary to utilize a large number of circuit boards in a system, such as in a telecommunications switching system, the speed of a bus system is reduced to an unacceptable extent. To overcome the problems of bus systems, ring interconnection systems have been proposed. (See, for example, J. M. Lenart, "A High Performance Satellite BaseBand Switching Technique," MILCOM 1984 Conference Record, Vol. 1, pp. 13-15 and J. M. Lenart et al, "A Ring Based Satellite Switch," Proc. from ICEC 1984, Vol. 1, pp. 216-218.) In a ring-connected switching system, signals are regenerated at each printed circuit board and are transmitted to the next adjacent circuit board. The connections are closed in a loop, or ring, so that signals propagate successively to each circuit board. At each circuit board, signals can be removed from the ring, introduced onto the ring or simply passed through the next circuit board on the ring. An advantage of the ring-connected system is that a large number of nodes can be utilized without reducing the system clock speed. In a ring-connected system having N nodes, or circuit boards, the time to transfer information from one circuit board to another is equal to or less than N clock cycles, the average being N/2 clock cycles. Each driver has only a single load in the ring-connected system regardless of the number of nodes.
To achieve high operating speed, it is necessary to minimize the distance between each circuit board. A conventional circuit board or card rack of parallel circuit boards has been undesirable for ring-connected systems since opposite ends of the card rack must be connected together to form a ring. Other circuit card configurations have been proposed to achieve the ring connection while minimizing wiring length. In one approach, a circular backplane is utilized and printed circuit boards are plugged in radially to form a spoke-type arrangement. This configuration achieves short interconnection lengths but is highly impractical, both as to construction and maintenance. A similar approach utilizes two conventional card racks mounted back-to-back with adjacent ends connected together to form an electrical ring. This configuration also achieves short interconnections but is impractical, since printed circuit boards must be accessed from two directions and the required space is relatively large. A third configuration utilizes two conventional card racks mounted one above the other with adjacent ends connected together. This configuration does not provide the desired short interconnections.
It is an object of the present invention to provide an improved, ring-connected circuit module assembly.
It is another object of the present invention to provide a ring-connected circuit module assembly utilizing a printed circuit board assembly having a single row of parallel circuit boards.
It is a further object of the present invention to provide a ring-connected circuit module assembly having short interconnections between circuit modules.
It is another object of the present invention to provide a ring-connected circuit module assembly wherein all circuit modules are accessible from one direction for easy servicing.
It is a further object of the present invention to provide a ring-connected circuit module assembly having relatively low cost and low space requirements.